Triode sputtering apparatus for depositing uniform coatings



Feb. 21, 1967 R. M. MOSESON 3,305,473

TRIODE SPUTTERING APPARATUS FOR DEPOSITING UNIFORM COATINGS Filed Aug. 20, 1964 United States Patent 9 3,305,473 TRIQDE SPUTTERING APPARATUS FOR DEPOSITING UNIFORM COATINGS Roger M. Moseson, Rochester, N.Y., assignor to Consolidated Vacuum Corporation, Rochester, N.Y., a corporation of New York Filed Aug. 20, 1964, Ser. No. 390,800 5 Claims. ((11. 204-298) The subject invention relates to the art of sputtering, and, more particularly, to apparatus for depositing thin films of material on a surface of a substrate by sputtering.

The phenomenon referred to as sputtering has been known for many years. Initially, this phenomenon was considered undesirable, since it caused blackening of tube Walls, poisoning of cathodes and other deleterious effects in gas discharge and high vacuum apparatus and devices. More recently, sputtering has been developed to a highly sophisticated technique which permits the deposition of thin layers of material on various substrates. To date, films of nearly all metallic elements and of many alloys have been deposited on substrates of insulating material, metal or metal alloys. With the advent of miniaturization in electronics and related fields, sputtering techniques have become particularly valuable.

The most common form of prior-art sputtering apparatus was composed of a plane cathode and a plane parallel anode located in an evacuated vessel which contained sufficient ionizable gas molecules to sustain an electric discharge. The cathode was formed of the material to be sputtered and the anode was positively biased with respect to the cathode to establish an ion plasma between the anode and cathode. The substrate on which a thin film of the cathode material was to be deposited was located on a Work support which was plane parallel to the cathode and was located between the anode and the cathode. Other forms of prior-art apparatus provided the anode in the form of an annular member located adjacent the cathode. The work support was then interposed between the anode member and an electrically biased ion accelerating electrode plate which extended parallel to the work support.

These prior-art apparatus had the disadvantage of requiring considerable energy for providing a sputtering action of a satisfactory order.

In more recent experiments, the material to be sputtered was provided in the form of an ion target which was located to the side of an ion plasma and was electrically biased to attract ions from the plasma. In this manner, considerable sputtering action at relatively low energies was realized. However, non-uniformities in the ion density of the plasma reflected itself in undesirable non-uniform depositions of sputtered material on the substrate, which is preferably disposed laterally of the ion plasma so as to face in the direction of the ion target. In my experiments, I have discovered ways and means of overcoming this disadvantage by means of magnetic fields, as will be apparent as the description proceeds.

The apparatus of the subject invention comprises an enclosure, means for evacuating the enclosure, and means for establishing in the enclosure an ion plasma extending substantially along a predetermined axis. An ion target of the material to be sputtered is located in the enclosure and has a surface spaced from and extending substantially parallel to the ion plasma axis. The ion target is electrically biased so that ions from the plasma will impinge on the ion target and sputter material therefrom. A substrate is mounted in the enclosure so that the surface of the substrate on which sputtered material is to be deposited faces the ion plasma axis and the above mentioned surface of the ion target. According to the invention, the apparatus includes means for establishing in the plasma may include an anode and a cathode.

3,305,473 Patented Feb. 21, 1967.

enclosure a magnetic field for controlling the density of ions at the target so as to achieve a uniform deposition of sputtered material on the substrate or a sputtered film having a desired gradient or gradients of thickness with respect to a plane through one surface of the film.

According to preferred embodiments, the magnetic field may be established to extend substantially along the above mentioned ion axis and may then be shifted along such axis until the desired uniformity or thickness gradient of the deposited film is reached. The magnetic field may be produced by one or more permanent magnets or by an electromagnetic coil or coils. In the latter case, the strength of the magnetic field may be varied by varying the energizing current of the coil or coils. The magnetic field may also be tilted with respect to the ion plasma axis or the ion target surface to control selectively the deposition of sputtered material on the substrate. In short, the magnetic field may be of controllable strength, orientation and location, if desired.

During the operation of the apparatus, thematerial sputtered from the ion target will deposit itself on the above mentioned surface of the substrate and form a thin film of the sputtered material thereon. In this manner, various metals and alloys may be deposited in the form of thin films on other metals or alloys or on bodies of insulating material. An undesirable obstruction of the ion plasma is avoided, since both the ion target and the substrate are located laterally of the plasma axis. Nevertheless, the ion-releasing plasma is still closely adjacent the target and the substrate. This results in increased sputtering efficiency. The means for establishing the above mentioned ion The cath ode is preferably in the form of an electrically heated filament serving as an electron source. This filament need not necessarily be of the same material as the ion target. In fact, it is preferable to select a filament material which is characterized by long life and high electron-releasing capacity. In this manner, the cathode is not consumed at the high rate which was prevalent in those prior-art apparatus in which the electron-releasing cathode was also the source of sputter material.

To establish the ion plasma, the anode is positively biased with respect to the electrically heated cathode. In a preferred embodiment of the invention, the anode has a surface which is spaced from and extends substantially at right angles to the target surface and the substrate surface.

The invention will become more readily apparent from the following detailed description of a preferred embodiment thereof, illustrated by way of example in the accompanying drawing, in which the figure shows an elevation,

partially in section, of an apparatus according to the in vention, with associated circuitry.

The sputtering and film-depositing apparatus 10 shown in the drawing comprises a base 11 having a central opening 12, and a removable bell jar 14 located on base 11 and sealed thereto at annular structure 15. A vacuum conduit 17 is connected to base 11 by a flange 1 8 and gasket 19. The space in base 11 and bell jar 14 is evacuated by means of a conventional high vacuum pump (not shown) which is connected to vacuum conduit 17.

The base 11 has a first lateral aperture 21 which communicates with the opening 12. A flange 22 is mounted to the base 11 at aperture 21 by a number of bolts 23. A sealing ring 25 seals the flange 22 to the base 11. A manually adjustable needle valve 26 is connected to flange 22 so that one of its ports communicates with aperture 21. The other port of needle valve 26 is connected to a pipe 27 which leads to a gas tank 28. The gas tank, which may be a gas bottle, contains an ionizable gas, such as argon, which has the effect of facilitating the ionization process in the bell jar 1-4, when admitted to the base 11 and jar 14 by the needle valve 26 in small quantities. It would also be possible to provide the necessary ionizing environment in bell jar 14 by adjusting the operation of the vacuum pump so as to permit a suflicient number of gas molecules to remain in the bell jar 14 during the evacuation process. However, the use of a separate gas tank and needle valve permits a more convenient and precise adjustment of the ionizing environment.

The base 11 has a second lateral aperture 31 which communicates with the space in bell jar 14 through a substantially tubular filament shield 32. A flange 33 is fastened to the base 11 at aperture 31 by a number of bolts 34. A sealing ring 36 seals the flange 33 to the base 11. Flange 33 has an extension 38 which forms the base of a cathode member 40. The cathode member further includes a filament 41 which is supported and supplied with electric current by a pair of leads 42 that extend through the flange 33 and are insulated therefrom by a pair of insulating sleeves 44. A pair of terminals 45 is mounted on sleeves 44 and connected to leads 42. In the illustrated embodiment, the filament shield 32 carries a coil of tubing 47 which is connected to a supply of coolant liquid in a conventional manner not shown per se. The coolant liquid is caused to circulate through tubing or coil 47 so as to cool the filament shield. In thi manner, overheating of the filament shield and the release of undesirable electrons and contaminating particles therefrom are avoided. A bafiie plate 50 encompasses the upper end of filament shield 32 and is supported by a pair of studs 51 and 52.

A bushing 54 extends through a bore 55 in flange 18 and is held therein by a nut 56. A tube 57 extends through and is sealed to the bushing 54. Tube 57 carries at its upper end an elbow member 59. A further tube 60 is connected to elbow member 59 to extend substantially perpendicularly to tube 57. An insulated wire 62 extends through tubes 57 and 60 and is connected to an anode support rod 64 at tube 60, which is insulated from tube 60, and to a terminal 65 which is mounted on the lower end of tube 57 by an insulator 66. An anode 68 is suspended from and electrically connected to support 64. Anode 68 has a substantially horizontal anode surface 70 which faces in the direction of filament shield 32.

After the base 11 and bell jar 14 have been evacuated, the filament 41 is supplied with a heating current from battery 72 which is connected to terminals 45. The anode 68 is then positively biased with respect to the filament 41 by a battery 73. The negative terminal of battery 73 is connected to one of the filament terminals 45, and may be grounded or connected to apparatus 10. The positive terminal of battery 73 is connected to anode terminal 65 through a variable resistor 75.

The heated filament 41 will release electrons to the anode surface 70. These electrons will collide with gas molecules present in bell jar 14. The gas molecules will thus be ionized and an ion plasma will form in the space between the anode and the baflle plate 50 near the filament or cathode. The ion plasma will extend substantially along an axi indicated by phantom line 77. The needle valve 26 may be adjusted from time to time to admit a desired number of gas molecules from the tank 28 to the ion plasma. Since the cathode is in the form of the heated filament 41, a large number of electrons will be released into the bell jar 14 and a vigorous formation of the ion plasma will take place. The ion plasma will thus be stronger than the plasmas obtained with the same energy in those prior-art apparatus in which the cathode was not of the hot filament type. Also, the cathode will be characterized by a long life, since it is not a source of sputtering material, as in prior-art apparatus, and can thus be formed of a typical cathode material that displays a high yield of electrons. The

heated cathode will have a low sputtering rate because of the low voltage drop between anode and cathode that can be realized with the apparatus of the subject invention.

The particular mounting of the cathode member 40 illustrated in the drawing is highly advantageous, since the cathode can be removed from a space laterally of the base 11, so that the seal between bell jar 14 and base 11 need not be broken and the apparatus need not be moved as in some prior-art structures. In addition, the illustrated arrangement of the cathode member 40 and the position of the filament shield 32 reduces to a minimum contamination of the space in bell jar 14 by cathode or filament material.

A further bushing extends through an aperture 81 in flange 18 and is held therein by a nut 82. A tube 84 extends through and is sealed to bushing 80. Tube 84 carries at its upper end an elbow member 85. A further tube 86 is connected to elbow member to extend substantially perpendicularly to tube 84. An insulated wire 88 extends through tubes 84 and 86 and is connected to a target support 89 located adjacent and insulated from tube 86. The lower end of wire 88 is electrically connected to a terminal 90 which is insulated from tube 84 by an insulator 92.

An ion target in the form of a plate 94 is mounted on target support 89. The plate 94 is of the material to be sputtered and has a surface 95 disposed laterally of the ion plasma axis 77. In the illustrated embodiment, the surface 95 is not only spaced from axis 77 but also extends substantially parallel thereto. A substrate support 96 is mounted on and extends from bafile plate 50. A substrate 98 is mounted on and suspended from support 96. The substrate 98 has a surface 100 on which a thin film of material sputtered from target surface 95 is to be deposited. The surface 100 is disposed laterally of the plasma axis 77 so that it is spaced from axis 77 and faces target surface 95. In the illustrated embodiment, the substrate surface 100 extends substantially parallel to the target surface 95 and the anode surface 70 is spaced from and extends substantially at right angles to the surfaces 95 and 100.

The negative terminal of. a battery 102 is connected to target terminal 90 through a variable resistor 103. The positive terminal of battery 102 is connected to the lead which extends from the positive terminal of battery 73. The tar-get 94 and surface 95 are thus electrically biased so that ions from the ion plasma are caused to impinge on target surface 95. These impinging ions will sputter material from the target surface 95, which material will deposit itself on the substrate surface v100 and form a thin film thereon. During operation of the illustrated apparatus, the anode current can be adjusted by means of resistor 75 and the target current by means of resistor 103.

The apparatus further includes a substantially cylindrical cover which rests on base 11 and is removable therefrom. The lateral wall of cover 110 may be of nonmagnetic wire mesh. A three-leg support 111 is placed on the top of cover 110. Three chains, two of which are apparent in the drawing at 112 and .114, are suspended from support 111. A magnet structure 115 which houses an electromagnetic coil 116 is suspended from the latter chains by a number of further chains 116 which are connected to the former chains by hooks 118.

The coil 1 16 is energized by a battery 121 through a variable resistor 120, which permits adjustment of the current through coil 116 and thus of the magnetic field produced by this coil. The coil is of annular configuration and the magnetic field, which is diagrammatically indicated in the drawing by arrow 123, extends primarily along and in the vicinity of axis 77.

In my experiments, I have discovered that the rate of deposition and thus the thickness of the film formed on substrate surface 100 can be varied at different points of the substrate surface by shifting the magnetic field produced by coil 116. The magnetic field can be selectively shifted along axis 77 toward and away from the anode 68 or anode surface 70, until a maximum uniformity of film thickness on substrate surface 100 is achieved. The occurrence of this optimum uniformity can be readily ascertained in a conventional manner by one of the well-known optical apparatus for the measurement of thin film thicknesses. These measuring apparatus and their use are well known in the art so that they have not been illustrated in the drawing. Shifting of the magnetic coil is easily effected in the illustrated embodiment by removing the hooks 118 from the one links of chains 112 and 114 and the third chain not visible in the drawing and by placing the hooks 118 into other links of the corresponding chains. If desired, other types of adjustable suspension, such as cable trains of variable length, may also be used to move coil 116. The magnetic field may also be easily tilted with respect to the axis 77 or the target surface 95 by placing one of the hooks 188 into a higher or lower link of the chains 112 and 114' than the other hooks. The intensity of the magnetic field may be varied by adjusting the variable resistor 120.

In one of my experiments with an apparatus of the type shown .in the drawings, I have observed the formation of high-quality films of sputtered material by employing an anode to cathode voltage of 40 volts and a target voltage of 800 volts. The filament voltage was about 16 volts and the filament current about 40 amperes. The anode current was in the vicinity of 3.7 amperes and the target current about 120 milliamperes. I used argon as an ionizing gas and adjusted its pressure to about one micron. The magnetic field was produced by a coil of about 23 inches in diameter and 40 turns of No. wire. The coil current was adjusted at about 14 amperes.

Those skilled in the art will realize that these values are merely indicated by way of example and that the drawing is only a somewhat schematic representation of one preferred embodiment of the invention. Many modifications within the scope of the subject invention are, of course, possible.

For example, the apparatus of the invention could also be arranged so that the plasma axis will extend in a horizontal, rather than a vertical direction. The batteries shown in the drawing could be replaced by conventional voltage and current supplies which may contain the customary transformers and rectifiers and, if desired, voltage or current stabilization circuits. Various conventional measuring instruments could be employed to measure the anode, filament and target currents and voltages. Also, the substrate may take the form of a component of an electronic circuit or part of any other body that lends itself to film deposition by sputtering. Other modifications will suggest themselves to those skilled in the art.

I claim:

1. Apparatus for depositing thin films of material on a surface of a substrate by sputtering, comprising:

(a) an enclosure;

(b) means for evacuating said enclosure and providing an ionizable atmosphere therein;

(c) an ion target of said material in said enclosure;

((1) means for mounting said substrate in the enclosure with said surface of the substrate extending substantially parallel to, being spaced from and facing, a surface of the target;

(e) means in said enclosure including an anode and a thermionic cathode for sustaining an electrical discharge between and generally axially parallel to the surface of said target and said substrate, said anode and said cathode being separate and independent from said substrate and said target;

(f) means for applying to said anode a positive elec tric potential with respect to said cathode for forming a substantially divergent ion plasma of random ionization between said cathode and said anode;

(g) means for applying a negative potential to said target to cause attraction of ions to said target;

(h) means for selectively establishing in said enclosure, at least during the absence of an ion plasma therein, a magnetic field having longitudinal field lines substantially parallel to the parallel surfaces of said substrate and said ion target, said field lines being unidirectional through the space between said substrate and said target for reshaping the divergent ion plasma to a controlled plasma of substantially uniform ionization in the space between the substrate surface and the ion target surface.

2. A three-element sputtering apparatus for forming a thin film comprising:

(a) an enclosure having a longitudinal axis;

(b) means for evacuating said enclosure and providing an ionizable atmosphere therein;

(c) an ion target located in said enclosure having a surface of a material to be sputtered with the sputtering surface spaced away from and facing said axis and being substantially parallel thereto;

(d) means for mounting a substrate in said enclosure and having a surface for receiving a film of sputtered material spaced away from and facing said axis, being substantially parallel thereto and substantially face to face with the ion target;

(e) means in said enclosure including an anode and a thermionic cathode for sustaining an electric discharge along said axis and generally parallel to the surface of said target and said substrate, said anode and said cathode being separate and independent from said substrate and said ion target;

(f) means for selectively establishing in said enclosure a magnetic field having longitudinal field lines along and substantially parallel to the axis and adjacent the sputtering surface of the ion target;

(g) means for applying to said anode a positive voltage with respect to said cathode for forming an ion plasma; and

(h) means applying an ion attracting voltage to said ion target for sputtering material from the sputtering surface of the target.

3. A sputtering apparatus in accordance with claim 2 wherein said field establishing means comprises at least one electromagnetic coil encircling a portion of said ion plasma and including means for tilting the coil and to shift the magnetic field lines away from the initial parallel position, said shifted field lines being operative in the presence of plasma to controllably vary the ion uniformity of said plasma adjacent said target surface.

4. A three-element sputtering apparatus for deposition of a thin film comprising:

(a) an enclosure having a longitudinal axis;

(b) means for evacuating said enclosure and providing an ionizable atmosphere therein;

(c) an ion tar-get located in said enclosure spaced away from said axis and having a surface of material to be sputtered with said surface facing said axis and being substantially parallel thereto;

(d) means for mounting a substrate in said enclosure and having a surface for receiving a film of sputtered material spaced away from and facing said axis, being substantially parallel thereto and substantially face to face with the ion target;

(e) means in said enclosure including an anode and a thermionic cathode for sustaining an electric discharge along said axis and generally parallel to the surface of said target and said substrate, said anode and said cathode being separate and independent from said substrate and said ion target;

(f) at least one electromagnetic coil surrounding said enclosure and substantially coaxial with said axis;

(g) means connected to said coil for selectively establishing magnetic field lines substantially parallel to said axis adjacent the sputtering surface of the ion target in the absence of any ion plasma in said enclosure;

(h) means for applying to said anode a positive volt- (h) means for applying a positive voltage to said anode age with respect to said cathode for forming a suband a negative voltage to said cathode for forming stantially diffuse plasma of ionization along said axis a substantially cone-shaped diffuse ion plasma along in the absence of said magnetic field, and for forming, said axis, the anode and cathode being arranged to in the presence of said magnetic field, a cylindrical sustain an electric discharge along said axis and genplasma of controlled ion uniformity in the space beerally parallel to the surface of said target and said tween the ion target surface and the substrate surface; substrate;

(i) means for applying an ion attracting voltage to said (i) means for applying a negative voltage to said ion ion target; target for attracting ions from said plasma to said (j) a cathode shield located in said enclosure and de- 10 target; and

fining an aperture on said axis for passing electrons to (j) at least one electromagnetic coil encircling a porthe anode and for shielding the substrate and the tion of said ion plasma and having field lines subtarget from contamination by material emitted from stantially parallel to the axis for reshaping the diffuse the cathode. plasma to a substantially cylindrical shaped plasma 5. A three-element, sputtering apparatus for deposition of controllable ion uniformity in the area between of thin films comprising: said ion target surface and said substrate surface.

(a) an enclosure;

(b) a thermionic cathode for emitting electrons; References Cited y the Examiner (c) an electron-impervious cathode shield having an UNITED STATES PATENTS electron-passing aperture communicating with said enclosure for directing electrons emitted from said g gfii z'z'i "5 3255;? cathode into said enclosure along an axis passing subu ar e a 2,239,642 4/ 1941 Burkhardt et al. 204298 X stantrally through the center of the aperture and 3 071271 2/1962 W h 204 192 running longitudinal to said enclosure; 5 55 5/1964 (d) an anode, having a surface area substantially largems er than the aperture area of the cathode shield, for OTHER REFERENCES collecting electrons emitted by said cathode; Kay Eric. of Applied Physics VOL 34 Number 4 (e) means for evacuating said enclosure and providing (Part I) April 1963, pages an lomzable atmosphere of about one mlcron sald Wehner: Advances in Electronics and Electron Physics,

enc1ure; vol. VII (edited by Marton, L., Academic Press), pages (f) an ion target, located in said enclosure independent of and separated from said anode and Sald cathode Stuart et al.: Journal of Applied Physics, vol. 33, No. 7,

said ion target being of a material to be sputtered July 1962, pages 2345 2352 with a surface spaced away from and facing said axis and being substantially arallel thereto; 35 References Cited by the Applicant (g) means for mounting a substrate in said enclosure UNITED STATES PATENTS independent of and separated from said target, said 2,157,478 5/1939 Burkhardt et al, anode and said cathode, for receiving a film of sput- 2,206,020 7/ 1940 Berghaus et a1.

tered material, said substrate having a surface spaced 2,636,855 4/ 1953 Schwarz. away from and facing said axis, being substantially parallel thereto and substantially face to face with JOHN MACK, Primary Examinerthe ion target; R. K. MIHALEK, Assistant Examiner. 

1. APPARATUS FOR DEPOSITING THIN FILMS OF MATERIAL ON A SURFACE OF A SUBSTRATE BY SPUTTERING COMPRISING: (A) AN ENCLOSURE; (B) MEANS FOR EVACUATING SAID ENCLOSURE AND PROVIDING AN IONIZABLE ATMOSPHERE THEREIN; (C) AN ION TARGET OF SAID MATERIAL IN SAID ENCLOSURE; (D) MEANS FOR MOUNTING SAID SUBSTRATE IN THE ENCLOSURE WITH SAID SURFACE OF THE SUBSTRATE EXTENDING SUBSTANTIALLY PARALLEL TO, BEING SPACED FROM AND FACING A SURFACE OF THE TARGET; (E) MEANS IN SAID ENCLOSURE INCLUDING AN ANODE AND A THERMIONIC CAHODE FOR SUSTAINING AN ELECTRICAL DISCHARGE BETWEEN AND GENERALLY AXIALLY PARALLEL TO THE SURFACE OF SAID TARGET AND SAID SUBSTRATE, SAID ANODE AND SAID CATHODE BEING SEPARATE AND INDEPENDENT FROM SAID SUBSTRATE AND SAID TARGET; (F) MEANS FOR APPLYING TO SAID ANODE A POSITIVE ELECTRIC POTENTIAL WITH RESPECT TO SAID CATHODE FOR FORMING A SUBSTANTIALLY DIVERGENT ION PLASMA OF RANDOM IONIZATION BETWEEN SAID CATHODE AND SAID ANODE; (G) MEANS FOR APPLYING A NEGATIVE POTENTIAL TO SAID TARGET TO CAUSE ATTRACTION OF IONS TO SAID TARGET; (H) MEANS FOR SELECTIVELY ESTABLISHING IN SAID ENCLOSURE, AT LEAST DURING THE ABSENCE OF AN ION PLASMA THEREIN, A MAGNETIC FIELD HAVING LONGITUDINAL FIELD LINES SUBSTANTIALLY PARALLEL TO BE PARALLEL SURFACES OF SAID SUBSTRATE AND SAID ION TARGET, SAID FIELD LINES BEING UNIDIRECTIONAL THROUGH THE SPACE BETWEEN AND SUBSTRATE AND SAID TARGET FOR RESHAPING THE DIVERGENT ION PLASMA TO A CONTROLLED PLASMA OF SUBSTANTIALLY UNIFORM IONIZATION IN THE SPACE BETWEEN THE SUBSTRATE SURFACE AND THE ION TARGET SURFACE. 